The assignees have developed a linear tape open format (LTO) data storage device, having a drive mechanism, and a linear tape data storage medium stored on a removable cartridge. In such LTO format devices, in order to begin writing data to the tape, the tape drive device must locate a suitable point along the tape data storage medium from which to start writing data. In the case of a tape data storage medium already having data written on it, the LTO format uses a longitudinal position data marker (LPOS) to locate an end of data already written to tape.
In the LTO format, there are recorded a plurality of parallel data tracks, running along a main length of the tape as well as a plurality of set of servo tracks running in parallel with the data tracks. As the tape moves past the read head, pulses from the servo track are counted, which gives an indication of the position of the read head along the length of the tape. This position is referred to as the longitudinal position (LPOS).
However, using the longitudinal position marker, the accuracy of positioning the tape head with respect to the linear tape is of the order of several code word quad (CQ) i.e. a set of two code word pairs, sets, and is close to the maximum append latency allowed by the format. This may lead to a format violation or another reposition operation, causing inefficiency.
In the LTO format the LPOS data gives a position along the tape for individual C1 code word pairs. However, the same position can apply to a plurality of consecutive C1 code word pairs and is not accurate down to the C1 code word pair level. Therefore, for writing data to the tape to append the existing data, the write head on the tape drive mechanism needs to be positioned at a position along the linear tape at the end of a current recorded data, in order to start a write or overwrite operation of new data onto the tape. The problem with the linear position data, is that because one LPOS data can apply to a plurality of sequentially written C1 code word pairs already on the tape, there is ambiguity on where to start writing. Therefore the LTO format skips to the next vacant LPOS, which incurs wastage of tape medium since writing may not start immediately after the last C1 code word pair in a data set.
Since up to 4 C1 code word quads can fall within the same LPOS number, a more accurate system is required.
The method of locating an end of data position longitudinally along a tape data storage medium is relatively inaccurate, compared to a length of tape occupied by a stored code word quad, which results in an unnecessarily high probability of format violations or tape reposition operations when searching for an end of data.